Spectrum search

The goal of the spectrum search is the identification of unknown substances based on a reference spectra database. In other words, to perform a search you need a library suited to your problem. The OPUS demo version offers two libraries containing 350 IR spectra and 246 Raman spectra, respectively. 

On the first page of the Spectrum Search dialog box (Fig. 11.26a) you have again to specify the spectrum files for the search run. If the Show Search Report immediately checkbox is selected, a window displaying the results opens auto-matieally after the search run. You can limit the search also to an existing search report (see Fig. 11.26b). In this case only the substances listed in the report will be compared to the test spectrum. 

On the Search Parameter page (Fig. 11.27) you define the desired search parameters  

Spectrum Search Search Parameters Excluded Regions Select Libraries  

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Excellent if unJutown is in reference file. Poor if unknown spectrum contains contaminant peaks. Better than whole spectrum search for Impure spectra. 

A PC-based 1H-nmr database, which includes full spectrum search capability, is being constructed by the Toyohashi University of Technology (67). Speclnfo, owned by Chemical Concepts, offers a 150,000 spectra library and database system for mainframe computers, which includes H, 15n, 19f, 17o, 31P-nmr, and a large collection of 13C-nmr spectra compiled by Bremser at BASF (68,69). It also offers nB-nmr spectra compiled by Nu th at the University of Munich. 

Chromatogram and spectrum search for IR, UV, Raman, nuclear magnetic resonance (NMR), MS, among others. 

Curve Eit A Integration Eeak Picking Quick Identity Test A Spectrum Search 3b Library Editor Library Browser... 

Figure 11.26. The Spectrum Search dialog box Use Search Report for Searching (a) not checked and (b) checked.

Figure 11.27. The Spectrum Search dialog box Search Parameters page.

The result of a spectrum search will be saved in the report data block sow>chI. By right-clicking on this block you have access to a short report of spectrum research an example is given in Fig. 11.30. By double-clicking on the search block you can open an extended report in a Library Window Hu, yWnd.. If you check the Show Search Report Immediately box on the Spectrum Search page, the extended report will be displayed automatically. 

C OPUSDEMO DATA MIRiUnl[Pg.152]

B Search Report AB i Report of Spectrum Search Number of Hits 1. (jerreral Ubrary IR UserLbrary 5 ... 

Figure 11.30. The short report of the spectrum search for the sample Unknown 4 based on the standard algorithm with the following parameters search sensitivity 14, maximum numbers of hits 5, minimum hit quality 300, and no excluded regions.

Figure 11.32. The extended report of a spectrum search with two spectra shown.

Perform a general spectrum search and then assign every peak (as far as possible) to nuclides using data held in a general library. As we will see, nuclide identification can be a complicated matter and in many cases an overall search and identification such as this would seem to be over the top . 

Peak search index. In a spectrum search, it is obviously important that as many peaks as possible should be detected but equally that as few spurious peaks as possible are reported. The index used by Keyser (1990) would seem to be useful ... 

NIST number The NIST number identifies a unique spectrum. Search used to retrieve such a spectrum for repeated examination. 

Although several variations in structure verification procedures are possible, the following sequence of steps provides a high level of confidence (Figure 1). Using the observed spectral data as the query, an identity spectrum search is initially conducted (step 1). If a single acceptable match results (2), its corresponding structure is retrieved and compared with the proposed structure (3), If the structures are identical, it can be considered that the proposed structure has been verified. However, since the breadth of the spectral search is limited... 

Another thing to consider when setting up a library search are the parts of the spectrum to include in the search. By default a full spectrum search uses the entire spectrum. However, some FTIR search software packages allow you to include or exclude regions of a spectrum from a search. This is a good idea if there are spectral regions that contain large water vapor and CO2 peaks, lots of noise, artifacts, or baseline problems. For example, if a mixture spectrum contains a known component, its peaks can be eliminated from the search to more effectively identify the unknown components in the sample. 

Full Spectrum Search In library searching, the use of the entire spectrum when comparing unknown and library spectra. 

Full spectrum search methods use, as the name implies, a fully digitized version of the spectrum. When used in this form, the spectra are usually stored at reduced resolution because at full resolution the storage overhead and the time required for processing become excessive. The matching of the unknown spectrum is usually based on a correlation criterion, such as the Dot Product between the library spectrum and the unknown. In practice, the only real advantage of this method is the availability of a digitized spectrum for display purposes. 

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